Device and method for the optically exciting laser-active crystals with polarization-dependent absorption

ABSTRACT

A device for the optical excitation of laser-active crystals with a diode laser ( 1 ) is disclosed. The diode laser ( 1 ) generates pump radiation ( 2 ), and the laser-active crystal ( 14 ) is arranged in a solid-state laser or solid-state laser amplifier. The laser-active crystal ( 14 ) has an axis (C) with strong absorption and an axis (A) with weak absorption. The pump radiation ( 2 ) from the diode laser ( 1 ) is substantially polarised linearly in a privileged polarisation direction. The device is configured in such a way that the pump radiation ( 2 ) is injected into the laser-active crystal ( 14 ) with a polarisation direction which is oriented parallel to the weak-absorption axis (A). The polarisation of the pump radiation in the vicinity of the laser-active crystal is oriented parallel relative to the weak-absorption axis.

The invention relates to a device for the optical excitation oflaser-active crystals with polarisation-dependent absorption, the devicehaving a diode laser which generates pump radiation, and a laser-activecrystal which is arranged in a solid-state laser or solid-state laseramplifier, the laser-active crystal having an axis with strongabsorption and an axis with weak absorption.

The invention furthermore relates to a method for the optical excitationof laser-active crystals with a diode laser, a laser-active crystalbeing arranged in a solid-state laser or solid-state laser amplifier.

Diode-pumped solid-state lasers, in which the diode-laser light isinjected as pump radiation into the laser-active crystal from the endface, have a high efficiency for converting the power of the pump light,which excites the laser-active crystal, into power of the laserradiation which is generated. The lasers are preferably used forgenerating laser light with low and medium powers (that is to say withoutput powers of about 1 to 100 W). Furthermore, these lasers areparticularly advantageous when the generated laser radiation is intendedto have a good beam quality and a high brilliance, that is to say a highspatial power density. Owing to these advantages, this type of laser issuccessfully used in many fields of research and technology.

The described excitation of the laser crystal with the diode-laserradiation, which is referred to as longitudinal excitation orend-pumping, allows optimum overlap of the volume which is opticallyexcited in the crystal and the volume of the modes of the laserresonator. A greatest possible part of the pump radiation absorbed inthe laser-active crystal is therefore converted into laser radiation.

The resonator modes determine the spatial properties of the laser beamwhich is generated. The latter typically has a circular or ellipticalcross section. The transverse fundamental mode of the resonator with aGaussian intensity distribution is preferred. In order to achieve a highspatial overlap between the pumped crystal volume and the resonatormodes, the distribution of the diode-laser radiation must first bespatially shaped so that the intensity distribution resulting from thiscan subsequently be imaged onto the spatial structure of the modes ofthe laser resonator.

High-power diode lasers which generate pump powers of typically inexcess of 4 watts are used for the excitation. According to the state ofthe art they consist of many individual emitters, which are arrangedbeside one another to form a so-called “bar” or “array” (“diode laserbar”). The emission geometry of such a pump-light source consisting of adiode laser is very different from a circular intensity distribution.The polarised pump radiation is emitted from the diode-laser bar in alinear cross section with a height of typically 1 μm and a width oftypically 10 mm. The emitted intensity distribution is almost Gaussian,although strongly divergent in the height direction of the emitting face(“fast axis”), and highly structured (diffraction factor M²>1000) alongthe width of the emitting face (“slow axis”). The radiation emitted bysuch diode lasers is thus unsuitable for direct longitudinal excitationof a solid-state laser. Suitable beam shaping of the pump light emittedby a laser bar is therefore necessary.

The beam shaping of the radiation coming from the diode laser ispreferably carried out using optical fibres with a diameter of from 0.2to 1 mm. The pump radiation from the diode laser is delivered to thelaser-active crystal by these light guides. There are several optionsfor injecting the diode-laser radiation into the optical fibres.Specification WO 98/35411 discloses at least one laser diode and awaveguide, which is arranged in front of the emitting face of the laserdiode. The radiation is injected into the waveguide by suitable opticalmeans, and can then be sent in a shaped form by the waveguide forfurther processing. Waveguides are likewise described in SpecificationWO 96/38749. Specification DE 100 12 480 discloses the use ofmicro-optics in order to shape the pump radiation coming from diodelasers.

The pump radiation incident in the laser-active crystal is absorbedthere by the laser-active ions. After entering the laser-active crystal,the intensity of the incident pump radiation is exponentially reduced inthe propagation direction by absorption. A significant proportion(typically from 25 to 50%) is dissipated to the laser-active crystal inthe form of heat. This thermal energy leads to local heating of thelaser-active crystal. The advantages of laser-active crystals with weakdoping are described, for example, in Specification WO 00/31842. Whenweakly doped laser-active crystals are used, the absorbed pump radiationis distributed over a larger volume.

According to the teaching disclosed in that specification, for example,in Nd:YVO₄ the doping should amount to less than 0.5%. The dopingconcentration of laser-active crystals, however, is currently subject tofluctuations of typically 0.1% due to fabrication. In laser-activecrystals with 0.3% doping, for example, these fluctuations leadrelatively to significant deviations from the setpoint value of thedoping concentration, which may amount to more than 30%. Reliableusability of these laser-active crystals is thereby restricted to asignificant extent.

Another method of reducing the thermo-optical problems in solid-statelasers, which are excited by diode lasers using optical fibres, is touse fibre bundles instead of a single multimode optical fibre. In thisway, the intensity distribution of the pump light can be shaped. This isdescribed, for example, in Specification EP 0990283 and in SpecificationWO 96/34436. The thermal gradients due to variation along the absorptiondirection and the concomitant thermally induced mechanical stressesremain, however.

Beam shaping by optical fibres has—besides the saidadvantages—substantial disadvantages, however, which generally result indisadvantageous properties for the longitudinally excited solid-statelasers produced using it:

-   -   a) Input losses, which typically amount to between 25 and 40%,        occur during input into the optical fibre. Therefore, at most        75% of the diode-laser radiation can be used as pump radiation        to excite the solid-state laser.    -   b) Manufacture of the diode lasers with optical fibres leads to        high costs.    -   c) The transport of the radiation in the optical fibre is not        polarisation-preserving, that is to say polarised components of        the pump radiation from the diode laser are converted into other        polarisation states by being transported in the optical fibre.        The polarisation direction of the light at the output of the        optical fibre is not generally stable as a function of time. For        example, the polarisation direction varies when the fibre is        mechanically moved.

The partial depolarisation of the pump radiation and the temporalinstability of the polarisation state can lead to significant problemsin end-pumped solid-state lasers. This is true especially if theabsorption coefficients of the laser crystals being used depend on thepolarisation direction of the incident pump light, or if the lasercontains optical elements or surfaces whose transmission properties aredependent on the polarisation state of the pump light, and on itsstability.

The problems due to excitation with unpolarised pump radiation can bedivided into two categories.

1. There are known to be laser-active crystals whose absorption isindependent of the polarisation (for example neodymium-yttrium aluminiumgarnet (Nd:YAG)), and ones in which the value of the absorptioncoefficient for the pump radiation depends on the polarisation of thepump light with respect to the crystalline axes. These laser-activecrystals are optically anisotropic in respect of absorption. Examples oflaser-active crystals which absorb polarisation-dependently are known toinclude neodymium-yttrium vanadate (Nd:YVO₄) and neodymium-gadoliniumvanadate (Nd:GdVO₄) as well as neodymium-yttrium lithium fluoride(Nd:YLF), neodymium-lanthanum scandium borate (Nd:LSB) andneodymium-yttrium aluminium perovskite (Nd:YAlO₃). These materials havehigh absorption coefficients for the pump radiation, the value of whichdiffers greatly along the crystal axes. In Nd:YVO₄, for example, 809 nmpump radiation with a polarisation parallel to the c-axis of the crystalis absorbed about 4 to 6 times more strongly than pump radiation with apolarisation parallel to the a-axis. Owing to these differences betweenthe absorption coefficients, it is no longer possible to find an optimumdoping concentration or crystal length so that partially polarised lightis absorbed uniformly in the crystal. This differential absorption ofthe various polarisation components leads to additional mechanicalstress loads in the laser crystal. Furthermore, a change in thepolarisation of the pump light causes a change in the absorbed power, aswell as its spatial distribution in the laser-active crystal. Laseroperation problems are generally caused in this way. The reasons forsuch problems are, for example, temporal fluctuations in thepolarisation direction, which are due for example to mechanicalmovements of the fibre or the pump-light source.

2. The pump radiation is conventionally injected into the solid-statelaser through resonator mirrors and the end faces of the laser-activecrystal. These elements are provided with dielectric coatings which havedefined reflection properties. They function as highly orsemi-reflective mirrors or as antireflection coatings, and they oftenhave polarisation-dependent transmission or reflection properties.Fluctuations in the pump-light polarisation can therefore lead tofluctuations in the transmitted pump power. Furthermore, it is common touse laser-active crystals whose end faces are oriented at the Brewsterangle, and which therefore have a polarisation-dependent transmission.Such crystal faces are transmissive with low loss only for a particularpolarisation direction.

In order to avoid the said disadvantages, methods have been developedwhich are intended to reduce or avoid a laser operation problem due to achange in the pump-light polarisation. However, the methods known fromthe prior art only partially resolve the described problems inend-pumped solid-state lasers. For instance, U.S. Pat. No. 6,137,820describes a method in which the pump radiation from the diode laser issplit into two subsidiary beams after being transported through anoptical fibre. The splitting is carried out with a polarizer, so thateach subsidiary beam is completely polarised linearly. The polarisedsubsidiary beams are used for the longitudinal excitation of asolid-state laser. However, the effect of polarisation changes in theoptical fibre is to change the power components in the two subsidiarybeams when the polarisation components are split at the polarizer. Thesepower fluctuations furthermore lead to spatial differences of theexcitation in the laser crystal, and therefore to changes in the laseroutput power and the beam quality. Another disadvantage is the reduction(halving) of the power of the pump radiation by the beam splitting.

U.S. Pat. No. 5,999,544 describes an arrangement in which the pumpradiation for a diode-pumped solid-state laser is deliberatelydepolarised before or after it is transported through an optical fibre.This avoids fluctuations in the polarisation state of the pumpradiation, and therefore minimises their effect on the laser operation.For laser-active crystals with anisotropic absorption, for exampleNd:YVO₄, one half of the unpolarised pump radiation is absorbed over ashort length and the other half is absorbed over a distance severaltimes longer. A significant part of the pump power is absorbed in theimmediate vicinity of the end face, where it locally causes strongheating of the laser-active crystal. The fact that the heat is notdistributed uniformly in the laser-active crystal causes significantthermo-optical problems. Another disadvantage is that the depolarisedpump radiation experiences significant losses when passing through faceswith a polarisation-dependent transmission (for example crystals withBrewster end faces).

It is an object of the invention to provide a device for the opticalexcitation of laser-active crystals with polarisation-dependentabsorption, which delivers constant laser radiation as a function oftime while efficiently utilising the pump radiation. Laser radiationwith a high efficiency and therefore a high output power with a goodspatial beam quality is also intended to be facilitated.

The object is achieved by a device which is characterised in that thepump radiation (2) from the diode laser (1) is substantially polarisedlinearly in a privileged polarisation direction, and in that thepolarisation direction of the pump radiation (2) is oriented parallel tothe weak-absorption axis (A) of the laser-active crystal (14) when it isincident in the laser-active crystal (14).

It is also an object of the invention to provide a method for theoptical excitation of laser-active crystals with polarisation-dependentabsorption, which delivers constant laser radiation as a function oftime while efficiently utilising the pump radiation. Laser radiationwith a high efficiency and therefore a high output power with a goodspatial beam quality is also intended to be facilitated.

The object is achieved by a method which is characterised by thefollowing steps:

-   -   shaping pump radiation generated by the diode laser, the shaped        pump radiation having a polarisation direction, and    -   projection onto a laser-active crystal, which has an axis with        strong absorption and an axis with weak absorption, so that the        polarisation direction of the pump radiation is oriented        parallel to the weak-absorption axis of the laser-active        crystal.

One advantage of the invention is that the absorbed power is therebydistributed over a substantially larger volume in the laser-activecrystal. Thermally induced effects in the laser-active crystal aretherefore substantially reduced. The power absorbed in the laser-activecrystal is distributed over a substantially larger crystal volume thanif the polarisation of the pump radiation were to be oriented so thatthe laser-active crystal absorbs strongly. This significantly reducesdisadvantageous modifications of laser properties due to local heatingof the laser-active crystal.

Another advantage is that laser radiation can thereby be produced with ahigh efficiency and therefore a high output power with a good spatialbeam quality. The radiation generated by this method is preferablydiffraction-limited and should have a high brilliance. The power of thelaser radiation is furthermore maximally stable as a function of time,and insensitive to external effects. With the invention, the pump powerof diode-pumped solid-state lasers is increased by several timescompared with existing systems, before thermally induced problems occurin the laser-active crystal. In this way, either the output power of thesolid-state laser can be multiplied or, with the same output power, thebeam quality and brilliance of the solid-state laser radiation can beimproved significantly.

The invention also affords the advantage that laser-active crystals withcommercially available doping can be used in solid-state lasers with ahigh output power. These crystals which, for example, contain more than0.5% (at.) neodymium, on the one hand offer the advantage that thefluctuations in the doping concentration due to production are lessserious than in crystals with weaker doping. On the other hand, morelaser energy can be stored per volume element in crystals with heavierdoping. This is, for example, particularly advantageous for lasers whichemit radiation in the form of light pulses. In general, the greateramount of stored energy makes it possible to generate more energeticlight pulses.

For the excitation of solid-state lasers, diode lasers which arearranged in the form of diode-laser bars (linearly or as atwo-dimensional matrix), with beam shaping by optical fibres, offer theadvantage that high pump powers (of up to 30 W) can be focused ontospots with a small diameter (typically from 0.4 to 1 mm). The focusedpump radiation has a circular cross section. Good overlap of the pumpedvolume of the laser-active crystal with the modes of the resonator ofthe solid-state laser can be achieved in this way. Owing to the use ofoptical fibres, the pump sources for diode lasers (including the powersupply and cooling) can furthermore be spatially separated from thelaser resonator, so that more compact and more stable laser structurescan be produced.

The pump radiation incident in the laser-active crystal is absorbedthere by the laser-active ions. After entering the laser-active crystal,the intensity of the incident pump radiation is exponentially reduced inthe propagation direction by absorption. The extent of this reduction,which is described by the absorption coefficients, depends on thespatial density of the laser-active ions (doping) and on theirabsorption cross section. Maximally high absorption is normally utilisedfor efficient excitation. Energy is stored in the laser-active ionsduring absorption, but only part of it is converted into laser light. Asignificant proportion (typically from 25 to 50%) is dissipated to thelaser-active crystal in the form of heat. This thermal energy leads tolocal heating of the laser-active crystal. Since many of thelaser-active crystals which are used have a comparatively poor thermalconductivity, significant temperature differences occur between theregions pumped to differing degrees in the laser-active crystal. Thetemperature differences lead to thermo-optical effects. For example, thecrystal end faces curve owing to the thermal expansion of the crystalcentre. The curved end faces act as a thick lens. The thermal expansionfurthermore generates mechanical stresses, which change the refractiveindex. The temperature profile which is formed inside the crystalresults in a refractive-index profile, which likewise acts as a lens.The sum of these effects, which are usually combined under the term“thermally induced lens”, leads to deformation of the emitted laser beamand usually deterioration of its spatial beam quality. The thermallyinduced mechanical stresses may even cause the crystals to fracture. Theinvention has the advantage of avoiding the aforementioned disadvantagesof the prior art.

Whereas the output power of diode-pumped solid-state lasers has beenrestricted in the past by the available pump power of the diode lasers,diode lasers are nowadays so powerful that a restriction is placed onthe output power by the thermal loading limit of the laser-activecrystals. With the invention, for the first time, this restriction doesnot occur until substantially higher pump powers, so that it is possibleto produce more powerful lasers with a good beam quality. The use ofpolarised pump radiation furthermore offers, for the first time, theopportunity to use optical components and resonator configurations forthe solid-state laser which could not previously be employed with partlyor unpolarised pump radiation from diode lasers, or could only be usedinefficiently.

Crystals which consist of Nd:YVO₄, Nd:GdVO₄, Nd:LSB, Nd:YAlO₃, Nd:YLF orNd:BEL have proved expedient as laser-active crystals. Other crystalswith anisotropic absorption may likewise be used.

It is important for the light from the diode laser to be imagedoptimally into the laser-active crystal. To that end, an optical elementwhich is configured in the form of micro-optics, for example, isarranged downstream of the diode laser in order to achieve appropriatespatial shaping of the pump radiation from the diode laser. It isparticularly advantageous for the optical element to be designed in theform of a waveguide, in order to achieve spatial shaping of the pumpradiation from the diode laser. The polarisation-dependent waveguidemay, for example, consist of a glass rod or an optical fibre.

In order to improve the output power of the solid-state laser in whichthe laser-active crystal is located, it is possible to provide aplurality of diode lasers which project the light of the pump radiationleaving them onto the laser-active crystal. At least one resonatormirror, for example, may be provided in order to project the pumpradiation onto the laser-active crystal. It is likewise conceivable forthe pump radiation, shaped suitably for the injection, to be injecteddirectly into the laser-active crystal.

Another preferred embodiment of the invention is, that the second endface of the laser-active crystal is assigned a reflector, which reflectsthe unabsorbed pump radiation that was injected through the first endface, and injects it into the second end face as reflected pumpradiation.

The laser-active crystal used here is selected such that it has dopingand a length so that approximately 50 to 60% of the pump radiation canbe absorbed in the laser-active crystal after entering through the firstend face.

In another preferred embodiment of the invention, a laser oscillatorwhich generates an output beam is provided. An input mirror for theoutput beam is provided between imaging optics for the pump beam and thefirst end face of the laser-active crystal. The output beam passesthrough the laser-active crystal at least once, while generating a beamwith higher output power.

Further advantageous refinements of the invention can be found in thedependent claims.

The subject-matter of the invention is schematically represented in thedrawing and will be described below with reference to the figures, inwhich:

FIG. 1 shows a schematic representation of the principle of thearrangement according to the invention;

FIG. 2 shows a schematic representation of an arrangement for shapingthe pump radiation of a diode laser and projecting it onto alaser-active crystal;

FIG. 3 shows a diagram, which shows the intensity of the light of thepump radiation as a function of the position in the laser-activecrystal;

FIG. 4 a shows a representation of the temperature distribution in thelaser-active crystal, the pump radiation being polarised in thec-direction of the laser-active crystal,

FIG. 4 b shows a representation of the temperature distribution in thelaser-active crystal, the pump radiation being polarised in thea-direction of the laser-active crystal,

FIG. 5 shows a schematic representation of a laser which generates thelaser radiation in the laser-active crystal of the pump radiation fromthe diode laser,

FIG. 6 shows an embodiment of the invention in which the pump radiationfrom a plurality of diode lasers is injected into the laser-activecrystal,

FIG. 7 shows a schematic representation of an embodiment of theinvention in which the pump radiation from a diode laser is injecteddirectly into a resonator provided with the laser-active crystal;

FIG. 8 a shows a diagram, which shows the intensity of the pumpradiation in the laser-active crystal with excitation through the firstand second end faces and polarisation in the direction of the a-axis;

FIG. 8 b shows a diagram, which shows the intensity of the pumpradiation in the laser-active crystal with excitation through the firstand second end faces using unpolarised light;

FIG. 9 shows a schematic representation of an embodiment of theinvention for reducing the local temperature differences withback-reflection of pump light;

FIG. 10 shows a graphical representation of the power density of theinjected pump radiation for the case with back-reflection of pump light;

FIG. 11 shows a pump arrangement for solid-state laser crystals with aplurality of end faces, which are arranged at the Brewster angle; and

FIG. 12 shows a schematic representation of a laser amplifier with thefeatures according to the invention, which is pumped by a diode laser.

The principle of the arrangement according to the invention isrepresented in FIG. 1. This arrangement contains a diode laser 1, whichemits linearly polarised light. The laser light is referred to as pumpradiation 2. The pump radiation 2 has its spatial distribution shapedspatially by an optical element 4, and is injected into a laser-activecrystal 14 which has a polarisation-dependent absorption. Thefundamental features of the arrangement according to the inventionincluding the use of linearly polarised pump radiation 2, which isgenerated by a diode laser 1. The linearly polarised pump radiation 2 isused for exciting (pumping) the laser-active crystal 14. Thelaser-active crystal 14 has a polarisation-dependent absorption, that isto say the pump radiation 2 is absorbed particularly well in apreferential crystal direction. The linear polarisation state of thepump radiation 2 should therefore be preserved as far as possible whenthe spatial distribution of the pump radiation 2 is being shaped by theoptical element 4. The spatial distribution shaped by the opticalelement 4 is, for example, suitable for longitudinal excitation of thesolid-state laser crystal. According to the invention, this laser-activecrystal 6 is oriented with respect to the direction of the linearlypolarised pump radiation so that the polarisation is parallel to thedirection of the weaker absorption of the laser-active crystal 14.

FIG. 2 shows a schematic representation of an arrangement for shapingthe pump radiation 2 from the diode laser 1. A preferred diode laser 1consists of a multiplicity of individual emitters. The individualemitters may be arranged in the form of a linear array. The arrangementmay likewise be configured in the form of a 2-dimensional array. Acoordinate system 8, with an x-direction X, a y-direction Y and az-direction Z, is also added to the representation in FIG. 1 in order toillustrate the spatial positioning of the arrangement and the opticaldirections. In the exemplary embodiment represented here, the diodelaser 1 has an exit facet 10 which is arranged parallel to they-direction Y. The polarised pump radiation 2, which has a strip-likeemission characteristic, is emitted in the z-direction Z from the exitfacet 10 by the diode laser 1. In a first x-y section plane 7immediately behind the diode laser 1, the intensity distribution of thispump radiation 2 has a dimension of typically 10 mm in the y-direction Yand a few μm in the x-direction X. The beam profile of the pumpradiation 2 is Gaussian and strongly divergent in the x-direction X(“fast axis”), and highly structured (diffraction factor M²>1000) in they-direction Y (“slow axis”). The pump radiation 2 is polarised with apolarisation ratio of about 100:1, typically in the y-direction Y. Thepolarisation direction of the pump radiation 2 is labelled by a doublearrow 7 a in the first x-y section plane 7. The beam shaping is carriedout so that the polarised pump radiation 2 from the diode laser 1 isshaped by at least one optical element 4, while preserving thepolarisation properties (linearly polarised), into a spatial intensitydistribution which can be imaged longitudinally onto the transversefundamental mode of a solid-state laser or laser amplifier. The pumpradiation 2, or the pump light, has an approximately circular,rectangular or elliptical intensity distribution in the a second x-ysection plane 9 behind the optical element 4. The pump radiation 2 isfurthermore linearly polarised, for example in the y-direction (aslabelled by the double arrow 9 a in the second x-y section plane 9), andthe polarisation ratio is preferably more than 10:1. In the exemplaryembodiment represented in FIG. 2, the polarisation directions in thefirst x-y section plane 7 and the second x-y section plane 9 arerepresented as being parallel. In the general case, the polarisationdirection in the second x-y section plane 9 may also be rotated withrespect to the first x-y section plane 7. What is important for theinvention is that the pump radiation 2 is linearly polarised andparallel to the weak-absorption axis A when it strikes the laser-activecrystal 14. For the optical shaping of the laser radiation, the opticalelement 4 may also be used in the form of polarisation-preservingmicro-optics, waveguides or polarisation-preserving optical fibres. Thespatially shaped pump radiation 2 is imaged onto a laser-active crystal14 by imaging optics 12. FIG. 2 represents a case in which thepolarisation direction is parallel to the weak-absorption axis A. Incases when this is not so, the laser-active crystal 14 may be orientedsuitably so as to satisfy parallelism of the weak-absorption axis A andthe polarisation direction. An optical means (not shown) may also beprovided, for example a phase plate or double reflection, which rotatesthe polarisation direction suitably so as to satisfy the requisiteparallelism. The laser-active crystal 14 has a first end face 14 a and asecond end face 14 b. The pump radiation 2 is injected into thelaser-active crystal 14 injected from at least one end face 14 a or 14b. The imaging optics 12 may be a lens or a lens system. Reflectiveoptics with imaging properties are also possible, for example reflectionand focusing of the pump radiation 2 by suitable mirrors. The absorptionby the laser-active crystal 14 is polarisation-dependent. The featuresresulting from this will be clarified below with reference to theexample of Nd:YVO₄. The diode laser 1, the optical element 4, theimaging optics 12 and the laser-active crystal 14 are arranged on acommon beam axis 11, which is represented by dots and dashes in FIG. 2.The laser-active crystal 14 is represented as being cylindrical with acircular cross section in FIG. 2. Other cross sections, for examplerectangular, square or polygonal, are nevertheless conceivable as well.

In a Nd:YVO₄ crystal which is used in the embodiment represented, thereis an optically privileged direction, which is perpendicular to the beamaxis 11 in the orientation described here. This privileged direction isreferred to as a crystalline c-axis c. The directions perpendicular toit are both referred to as crystalline a-axes a. For pump radiation 2with a wavelength of 808.6 nm, the maximum absorption coefficient forthe pump radiation 2 which is polarised parallel to the crystallinec-axis c is α_(c)=40.7 cm⁻¹. It is therefore about four times as greatas the absorption coefficient α_(a)=10.5 cm⁻¹ for radiation which ispolarised perpendicularly thereto, that is to say in the direction ofthe crystalline a-axis a. In the invention, the polarisation directionof the pump radiation 2 is oriented in the direction of the weakabsorption of the laser-active crystal 14. In the example shown in FIG.2, the orientation is in the y-direction Y of the coordinate system andtherefore parallel to the crystalline a-axis a of the laser-activecrystal 14.

The use of a material like Nd:YVO₄ as the material for the laser-activecrystal 14 is particularly advantageous since this material is capableof efficiently absorbing the pump radiation 2 having a definedpolarisation in the direction of the crystalline a-axis a(weak-absorption axis) with a comparatively low absorption coefficient,and of emitting laser radiation having polarisation perpendicular to it,that is to say parallel to the crystalline c-axis c (strong-absorptionaxis). The particular advantage of emission with polarisation parallelto the crystalline c-axis c is that this radiation has a high crosssection for stimulated emission, that is the say it experiences stronglaser amplification.

The reduction in the thermal load on the laser-active crystal 14, whichresults from the arrangement according to the invention, will besubstantiated below with reference to the example of a typical Nd:YVO₄crystal. The laser-active crystal 14 has a diameter of 4 mm, a length of8 mm and neodymium doping of 1% (at.). The pump radiation 2 has a powerof 10 watts and is injected into the laser-active crystal 14 from thefirst end face 14 a through a circular area with a diameter of 0.8 mm.For the actually effective absorption of this pump radiation 2, thewidths of the spectral distributions of pump light and the absorptionlines must be taken into account. The so-called “effective” absorptioncoefficients are calculated from the convolution integral of the twodistributions and, for example, for a typical diode laser 1 with aspectral width of 3 nm and a central wavelength of 808.6 nm they areα_(c)(eff.)=21.3 cm⁻¹ and α_(a)(eff.)=3.8 cm⁻¹. The effective absorptionis thus about six times stronger for pump radiation 2 which is polarisedin the direction of the crystalline c-axis c, than for pump radiation 2with polarisation in the direction of the crystalline a-axis a. FIG. 3represents the local power density (intensity) of the pump radiation 2in the laser-active crystal 14 along the z-direction Z. On the first endface 14 a of the laser-active crystal 14 (z=0), it is 2 kW/cm². Afterentering the laser active crystal 14, the intensity of the incident pumpradiation 2 decreases exponentially owing to the absorption. A firstcurve 16 shows the absorption for the pump radiation 2 which ispolarised parallel to the crystalline c-axis c. Even after only 0.47 mm,the intensity has dropped to 36% (1/e). In comparison, a second curve 18shows the absorption for the pump radiation 2 which is polarisedperpendicularly to the crystalline c-axis c, that is to say in thedirection of the weak-absorption axis A. The intensity has not droppedto 36% (1/e) until after 2.7 mm, that is to say after a distance aboutsix times longer. Over the 8 mm length of the laser-active crystal 14,95% of the light of the pump radiation 2 is absorbed. The absorption forunpolarised pump radiation 2 is represented by the dotted third curve20. The sharp drop in the vicinity of the first end face 14 a of thelaser-active crystal 14 can be seen clearly, showing that more than halfof the power is absorbed over a distance of less than 1 mm.

FIG. 4 a and FIG. 4 b represent the temperature increases anddistributions in the Nd:YVO₄ crystal, which are caused by the absorptionof the pump radiation 2. The temperature distributions were calculatedusing a suitable simulation model. In order to illustrate thetemperatures inside the laser-active crystal 14, a section on an x-zplane through the cylindrical laser-active crystal 14 is represented inan upper image part 22, and the temperature distribution on the firstend face 14 a of the laser-active crystal 14 can be seen in a lowerimage part 24. A temperature scale 26 is respectively represented nextto the upper and lower image parts 22 and 24. FIG. 4 a represents thetemperature increase due to the absorption of the pump radiation 2 witha power of 10 W, which is polarised in the c-direction C. The maximumtemperature increase is 99° C., and is located directly on the first endface 14 a of the laser-active crystal 14. This causes stresses of about40 MPa on the lateral surface of the laser-active crystal 14. This valueis already close to the fracture limit, which is about 50 MPa forNd:YVO₄. The results of the calculation are in good agreement withexperiments, in which a crystal with the parameters described hereusually broke at a pump power of about 13 watts. The same case isplotted in FIG. 4 b for the pump radiation 2 polarised in the a-axis A.The maximum temperature increase is only 33° C., and the heat isdistributed over a much larger volume. The thermally induced stressesare at most 15 MPa. For the laser crystal 6 described in this example,the pump power is thus increased by three times with the inventioncompared to previously conventional systems. A substantially higheroutput power of the solid-state laser can therefore be achieved.

In order to produce a laser, the laser-active crystal 14 is located asrepresented in FIG. 5 in a laser resonator 27, which is typically formedby a multiplicity of mirrors 28, 29, 30 or reflective faces. In thisexemplary embodiment, the mirrors 28, 29, 30 are an end mirror 28,optional folding mirror 29 and an output mirror 30. One or more of thesereflective coatings may also be located directly on the surface of thelaser-active crystal 14. The modes of the laser resonator 27 determinethe spatial properties of the laser beam which is produced. Thetransverse fundamental mode of the laser resonator 27 with a Gaussianintensity distribution is preferred. The pump radiation 2 is preferablyinjected into the laser-active crystal 14 through one or more of theresonator mirrors (28, 29, 30). The injection takes place along the beamaxis 11 (here the z-direction) or at a small angle to it.

In order to obtain higher output powers from the laser-active crystal14, pump radiation 2 may be simultaneously injected into thelaser-active crystal 14 through its first and second end faces 14 a and14 b (see FIG. 6). A plurality of diode lasers 1 are provided, whichproject the light of the pump radiation 2 leaving them onto thelaser-active crystal 14. As already described in FIG. 2, each diodelaser 1 is assigned an optical element 4 in order to shape the pumpradiation 2. Each optical element 4 is likewise assigned imaging optics12, which image the pump radiation 2 onto the laser-active crystal 14.At least one resonator mirror 30, 31 and 32 is provided for projectingthe pump radiation 2 onto the laser-active crystal 14. The light of thepump radiation 2 from a plurality of diode lasers 1 may furthermore beinjected simultaneously from at least one side of the crystal.

FIG. 7 discloses a further exemplary embodiment of the invention. Inorder to produce a laser, the laser-active crystal 14 is likewiselocated as represented in FIG. 5 in a laser resonator 27, which isformed here by a first resonator mirror 21 and a second resonator mirror23. The modes of the laser resonator 27 determine the spatial propertiesof the laser beam which is produced. The transverse fundamental mode ofthe laser resonator 27 with a Gaussian intensity distribution ispreferred. The pump radiation 2 is injected directly into the laserresonator 27 in this exemplary embodiment. To that end, an appropriateinput means 25 is provided, which projects the pump radiation onto thefirst end face 14 a of the laser-active crystal 14. As already describedin FIG. 2, each diode laser 1 is assigned an optical element 4 in orderto shape the pump radiation 2. Each optical element 4 is likewiseassigned imaging optics 12, which image the pump radiation 2 onto theinput element 25. The input element 25 is, for example, designed in theform of a polarisation beam splitter. Other forms of the input element25 are familiar to the person skilled in the art.

The sharp decrease of the incident pump radiation 2, and the concomitantnonuniform distribution of the heat, can only be compensated forconditionally by the excitation through both end faces 14 a or 14 b. Byanalogy with FIG. 3, FIGS. 8 a and 8 b represent the intensity of theincident pump light in the laser crystal along the z-direction Z, butfor excitation through the first and second end faces 14 a and 14 b. Byanalogy with FIG. 3, a first curve 35 in FIG. 8 a shows the absorptionfor pump radiation 2 which is polarised in the direction of theweak-absorption axis A and is injected through the first end face 14 a.A second curve 37 represents the corresponding intensity profile for thepump radiation 2 which is injected through the second end face 14 b. Theintensities are added to form a third curve 39, which represents thetotal intensity. The total intensity does not have a gradient in themiddle of the laser-active crystal 14. The decrease of the intensity inthe vicinity of the first and second end faces 14 a and 14 b of thelaser-active crystal 14, and the local temperature differences resultingtherefrom, are modified less. Even in this case, the arrangementaccording to the invention leads to a substantial reduction of thethermally induced effects. As a comparison with this, excitation withunpolarised pump radiation 2 is represented in FIG. 8 b. In FIG. 8 b, afirst curve 40 shows the absorption of the unpolarised pump radiation 2,which is injected through the first end face 14 a. Correspondingly, asecond curve 41 represents the intensity profile for the pump radiation2 which is injected through the second end face 14 b. The intensitiesare added to form a third curve 42, which represents the totalintensity. The sharp reduction of the total intensity in the vicinity ofthe first and second end faces 14 a and 14 b is particularly noticeablein FIG. 7 b. As already mentioned in FIG. 3, greatly differingtemperature distributions are generated in the laser-active crystal bythe sharp reduction, and the thermally induced problems therefore remainunchanged. Another method of reducing the local temperature differencesin a laser-active crystal 14 with polarisation-dependent absorption bythe arrangement according to the invention is to use the unabsorbed pumpradiation, which is represented in FIG. 9. In this method withback-reflection of the pump light, the polarised pump radiation 2 fromthe diode laser 1 is shaped in the optical element 4 and the imagingoptics 12, as described, and is injected into the laser-active crystal14. The doping and the length of the laser-active crystal 14 areselected so that only a part of the pump radiation 2, typically about 40to 60%, is absorbed in the laser-active crystal 14. Unabsorbed pumpradiation 50 is returned into the laser-active crystal 14 by a reflector52. The reflector 52 generates reflected pump radiation 54, which has adivergence suitable for excitation of the laser. This reflector 52 mayalso contain imaging optical elements, and it may be applied directlyonto the second end face 14 b of the laser-active crystal 14. Thereflected pump radiation 54 only passes through the laser-active crystal14 in the opposite direction to the pump radiation 2 injected throughthe first end face 14 a, and is further absorbed during this. A moreuniform distribution of the intensity of the pump radiation 2 in thelaser-active crystal 14 is achieved in this way. It is furthermorepossible to use shorter laser crystals, which is advantageous for thegeneration of ultra-short laser pulses.

FIG. 10 represents the power density of the incident pump radiation 2 inthe laser-active crystal 14 for the case of the pump-lightback-reflection described in FIG. 9. The laser-active Nd:YVO₄ crystalhas a length of 2 mm in this example. On the first end face 14 a of thelaser-active crystal 14 (z=0), the pump power density is 2 kW/cm². Whenpassing through the laser crystal, the intensity of the incident pumplight decreases to about 1 kW/cm² owing to the absorption, as shown in afirst curve 56. The unabsorbed pump radiation 50 is reflected back, andfurther absorbed on the return leg through the laser-active crystal 14,as represented in a second curve 57. The total intensity is representedin a third curve 58, and shows a very minor reduction in the intensityalong the incidence direction. In this way, the aforementioned thermallyinduced problems in the laser-active crystal 14 are further reduced orsubstantially avoided.

FIG. 11 shows the pump arrangement for solid-state laser crystals 60with one or more end faces 60 a, 60 b which are arranged at the Brewsterangle. The polarisation direction (here the y-direction Y) of the pumpradiation 2, that is to say the direction of the E-field vector 62represented by the double arrow, in this case lies in the plane of theBrewster section. The pump radiation 2 is generated by at least onediode laser 1. As represented in FIG. 5, FIG. 6 or FIG. 9, the opticalelement 4 and the imaging optics 12 are arranged downstream of the diodelaser 1. The pump radiation 2 is injected through a first folding mirror64 or past a second folding mirror 66. In the event that thepolarisation of the pump radiation 2 and the laser light are orientedperpendicular to each other, the pump radiation 2 may also be injectedinto the solid-state laser crystal 60 via an optical element withpolarisation-dependent transmission and reflection (for example apolarizer) internal to the resonator. The solid-state laser crystal 60is likewise located in a resonator 67, which is bounded by a firstresonator mirror 68 and a second resonator mirror 69. Such anarrangement can offer significant advantages. On the one hand, it ispossible to produce a geometrical resonator configuration in which thesolid-state laser crystal 60 does not need to be placed in the vicinityof a resonator mirror 68 or 69. On the other hand, it is possible toproduce laser systems in which injection of the pump radiation 2 throughthe folding mirrors 64 or 66 would be difficult or disadvantageous. Forexample, this is the case with UV light excitation for which the mirrorshave a low damage threshold, or in the event that the pump radiation 2and the laser light have a similar wavelength.

FIG. 12 shows the arrangement with the features according to theinvention for a laser amplifier, which consists of a diode laser 1 asthe pump-light source, an optical element 4, imaging optics 12 and alaser-active crystal 14. The pump radiation 2 absorbed in thelaser-active crystal 14 leads to energy storage there. A laseroscillator 70 generates an output beam 71, the beam profile of whichpreferably corresponds to the transverse fundamental mode.

The power of the laser oscillator 70 may be output continuously or in apulsed form. An input mirror 74 is provided in order to inject theoutput beam 71 of the laser oscillator 70 into the laser-active crystal14. The output beam 71 of the laser oscillator 70 passes through thelaser-active crystal 14 one or more times. The energy stored in thelaser-active crystal 14 amplifies the output beam 71 to form a beam withhigher output power 72. Care should be taken that the spatial shape ofthe pump radiation is matched to the profile of the output beam 71 ofthe laser oscillator 70. The orientation of the linearly polarised pumpradiation 2 from the diode laser 1 parallel to the direction of theweaker absorption (a-axis A) of the laser-active crystal 14, accordingto the invention, achieves distribution of the power absorbed in thelaser-active crystal 14 over a substantially larger crystal volume. Asdescribed with reference to FIG. 1, less thermally induced problems arethereby caused in the laser crystal. In this way, the beam profile ofthe amplified beam is substantially not modified and a high spatialquality of the beam from the laser oscillator 70 is maintained evenduring the amplification.

The input mirror 74 may also be omitted in the present exemplaryembodiment. The output beam 71 of the laser oscillator 70 will then beinjected into the laser-active crystal 14 directly via the first orsecond end face 14 a or 14 b.

The invention has been described with reference to a particularembodiment. It is of course clear that changes and modifications may bemade, without thereby departing from the scope of protection of theclaims which follow.

LIST OF REFERENCES

-   1 diode laser-   2 pump radiation-   4 optical element-   7 first x-y section plane-   7 a double arrow-   8 coordinate system-   9 second x-y section plane-   9 a double arrow-   10 exit facet-   11 beam axis-   12 imaging optics-   14 laser-active crystal-   14 a first end face-   14 b second end face-   16 first curve-   18 second curve-   20 third curve-   21 first resonator mirror-   22 upper image part-   23 second resonator mirror-   24 lower image part-   25 input means-   26 temperature scale-   27 laser resonator-   28 end mirror-   29 folding mirror-   30 output mirror-   31 resonator mirror-   32 resonator mirror-   33 resonator mirror-   35 first curve-   37 second curve-   39 third curve-   40 first curve-   41 second curve-   42 third curve-   50 unabsorbed pump radiation-   52 reflector-   54 reflected pump radiation-   56 first curve-   57 second curve-   58 third curve-   60 solid-state laser crystal-   60 a end face-   60 b end face-   62 E-field vector-   64 first folding mirror-   66 second folding mirror-   67 resonator-   68 first resonator mirror-   69 second resonator mirror-   70 laser oscillator-   71 output beam-   72 amplified output beam-   74 input mirror-   A axis with weak absorption-   C axis with strong absorption-   a crystalline a-axis-   c crystalline c-axis-   X x-direction-   Y y-direction-   Z z-direction

1. Device for the optical excitation of laser-active crystals, with adiode laser (1) which generates pump radiation (2), the laser-activecrystal being arranged in a solid-state laser or solid-state laseramplifier and the laser-active crystal having an axis (C) with strongabsorption and an axis (A) with weak absorption, comprising: an opticalelement (4) is arranged downstream of the diode laser (1) in order toachieve spatial shaping of the pump radiation from the diode laser (1)and in that the pump radiation (2) from the diode laser (1) issubstantially polarised linearly in a privileged polarisation direction,and in that the polarisation direction of the pump radiation (2) isoriented parallel to the weak-absorption axis (A) of the laser-activecrystal (14) when it is incident in the laser-active crystal (14); andwherein the laser-active crystal (14) has at least a first and a secondend face (14 a, 14 b) which have a polarisation-dependent transmission,and in that the polarisation direction of the pump radiation (2) isoriented so that the reflection losses at the first or second end faces(14 a, 14 b) are minimal and the optical power which enters thelaser-active crystal (14) is maximal.
 2. Device for the opticalexcitation of laser-active crystals, with a diode laser (1) whichgenerates pump radiation (2), the laser-active crystal being arranged ina solid-state laser or solid-state laser amplifier and the laser-activecrystal having an axis (C) with strong absorption and an axis (A) withweak absorption, comprising: an optical element (4) is arrangeddownstream of the diode laser (1) in order to achieve spatial shaping ofthe pump radiation from the diode laser (1) and in that the pumpradiation (2) from the diode laser (1) is substantially polarisedlinearly in a privileged polarisation direction, and in that thepolarisation direction of the pump radiation (2) is oriented parallel tothe weak-absorption axis (A) of the laser-active crystal (14) when it isincident in the laser-active crystal (14); and wherein the solid-statelaser or solid-state laser amplifier comprises a laser resonator (27)with a multiplicity of mirrors (28, 29, 30), the surfaces of which areprovided with polarisation-dependent transmission, and in that thepolarisation direction of the pump radiation (2) is oriented so that thereflection losses at these surfaces are minimal and the optical powerwhich enters the laser-active crystal (14) is maximal.
 3. Deviceaccording to claim 2, wherein the laser-active crystal (14) consists ofNd:YV0 ₄, Nd:GdVO₄, Nd:LSB, Nd:YA10 ₃, Nd.:YLF or Nd:BEL.
 4. Deviceaccording to claim 2, wherein the laser-active crystal (14) consists ofNd:YV0 ₄ with neodymium doping of more than 0.5% (at.).
 5. Deviceaccording to claim 2, wherein the optical element (4) is configured inthe form of microoptics.
 6. Device for the optical excitation oflaser-active crystals, with a diode laser (1) which generates pumpradiation (2), the laser-active crystal being arranged in a solid-statelaser or solid-state laser amplifier and the laser-active crystal havingan axis (C) with strong absorption and an axis (A) with weak absorption,comprising: an optical element (4) is arranged downstream of the diodelaser (1) in order to achieve spatial shaping of the pump radiation fromthe diode laser (1) and in that the pump radiation (2) from the diodelaser (1) is substantially polarised linearly in a privilegedpolarisation direction, and in that the polarisation direction of thepump radiation (2) is oriented parallel to the weak-absorption axis (A)of the laser-active crystal (14) when it is incident in the laser-activecrystal (14); and wherein the optical element (4) is designed in theform of a polarisation-preserving waveguide, in order to achieve spatialshaping of the pump radiation (2) from the diode laser (1), thepolarisation-dependent waveguide consisting, for example, of a glass rodor an optical fibre.
 7. Device for the optical excitation oflaser-active crystals, with a diode laser (1) which generates pumpradiation (2), the laser-active crystal being arranged in a solid-statelaser or solidstate laser amplifier and the laser-active crystal havingan axis (C) with strong absorption and an axis (A) with weak absorption,comprising: an optical element (4) is arranged downstream of the diodelaser (1) in order to achieve spatial shaping of the pump radiation fromthe diode laser (1) and in that the pump radiation (2) from the diodelaser (1) is substantially polarised linearly in a privilegedpolarisation direction, and in that the polarisation direction of thepump radiation (2) is oriented parallel to the weak-absorption axis (A)of the laser-active crystal (14) when it is incident in the laser-activecrystal (14); wherein the second end face (14 b) of the laser-activecrystal (14) is assigned a reflector (52), which reflects the unabsorbedpump radiation (50) that was injected through the first end face (14 a),and injects it into the second end face (14 b) as reflected pumpradiation (54); and wherein the laser-active crystal (14) has doping anda length which are selected so that less than 70% of the pump radiation(2) can be absorbed in the laser-active crystal (14) after enteringthrough the first end face (14 a).
 8. Device according to claim 7,wherein approximately 50 to 60% of the pump radiation (2) can beabsorbed in the laser-active crystal (14) after entering through thefirst end face (14 a).
 9. Method for the optical excitation oflaser-active crystals with a diode laser (1), the laser-active crystal(14) being arranged in a solid-state laser or solid-state laseramplifier, comprising: spatially shaping pump radiation (2) generated bythe diode laser (1) with an optical element (4), the shaped pumpradiation (2) having a polarisation direction, and projection onto alaser-active crystal (14), which has an axis (C) with strong absorptionand an axis (A) with weak absorption, so that the polarisation directionof the pump radiation (2) is oriented parallel to the weak-absorptionaxis (A) of the laser-active crystal (14); and wherein the laser-activecrystal (14) has at least a first and a second end face (14 a, 14 b)which have a polarisation-dependent transmission, and in that thepolarisation direction of the pump radiation (2) is oriented so that thereflection losses at the first or second end faces (14 a, 14 b) areminimal and the optical power which enters the laser-active crystal (14)is maximal.
 10. Method for the optical excitation of laser-activecrystals with a diode laser (1), the laser-active crystal (14) beingarranged in a solid-state laser or solid-state laser amplifier,comprising: spatially shaping pump radiation (2) generated by the diodelaser (1) with an optical element (4), the shaped pump radiation (2)having a polarisation direction, and projection onto a laser-activecrystal (14), which has an axis (C) with strong absorption and an axis(A) with weak absorption, so that the polarisation direction of the pumpradiation (2) is oriented parallel to the weak-absorption axis (A) ofthe laser-active crystal (14); and wherein the solid-state laser orsolid-state laser amplifier comprises a laser resonator (27) with amultiplicity of mirrors (28, 29, 30), the surfaces of which are providedwith polarisation-dependent transmission, and in that the polarisationdirection of the pump radiation (2) is oriented so that the reflectionlosses at these surfaces are minimal and the optical power which entersthe laser-active crystal (14) is maximal.
 11. Method according to claim10, wherein the laser-active crystal (14) consists of Nd:YV0 ₄,Nd:GdVO₄, Nd:LSB, Nd:YA10 ₃, Nd:YLF or Nd:BEL.
 12. Method according toclaim 10, wherein the laser-active crystal (14) consists of Nd:YV0 ₄with neodymium doping of more than 0.5% (at.).
 13. Method for theoptical excitation of laser-active crystals with a diode laser (1), thelaser-active crystal (14) being arranged in a solid-state laser orsolid-state laser amplifier, comprising: spatially shaping pumpradiation (2) generated by the diode laser (1) with an optical element(4), the shaped pump radiation (2) having a polarisation direction, andprojection onto a laser-active crystal (14), which has an axis (C) withstrong absorption and an axis (A) with weak absorption, so that thepolarisation direction of the pump radiation (2) is oriented parallel tothe weak-absorption axis (A) of the laser-active crystal (14); whereinpump radiation (52) emerging from the second end face (14 b) of thelaser-active crystal (14) is reflected by a a reflector (52), andre-enters the laser active crystal (14) as reflected pump radiation (54)through the second end face (14 b); and wherein the laser-active crystal(14) has doping and a length which are selected so that less than 70% ofthe pump radiation (2) can be absorbed in the laser-active crystal (14)after entering through the first end face (14 a).
 14. Method accordingto claim 13, wherein approximately 50 to 60% of the pump radiation (2)is absorbed in the laser-active crystal (14) after entering through thefirst end face (14 a).